Mobile terminals were first developed to provide wireless communication between users. As technology has advanced, mobile terminals now provide many additional features beyond the simple telephone conversation. For example, mobile terminals are now able to provide advanced functions such as an alarm, a Short Messaging Service (SMS), a Multimedia Messaging Service (MMS), E-mail, games, short range communication, an image capturing function using a mounted digital camera, a multimedia function for providing audio and video content, a scheduling function, and many more. With the plurality of features now provided, a mobile terminal has effectively become a necessity of daily life for most people.
As part of the image capturing function, a processor of the mobile terminal receives image data from the mounted digital camera and ultimately controls to output a corresponding image on a display unit. In order to provide the image on the display unit, the data received from the digital camera must first undergo a series of manipulations.
FIG. 1 illustrates a captured image and a display unit for displaying an image corresponding to the captured image according to the prior art.
Referring to FIG. 1, an image 104 is captured by a digital camera of the mobile terminal. Raw data corresponding to the captured image 104 is manipulated such that it may be displayed (i.e., substantially reproduced) on a display unit 110. Image 104 is represented by a plurality of pixels 106, wherein each pixel 106 is associated with a color that should be substantially reproduced by the display 110. To display the image 104 on the display unit 110, each pixel 106 is mapped onto a set of one or more subpixels of the display unit 110, which then display the pixel's color.
In the prior art, each subpixel of the display unit 110 displays a “primary” color. That is, each subpixel is associated with a specific hue value and a specific saturation value. In some display units, each repeating set of subpixels includes a subpixel for each primary color. The subpixels are small and spaced closely together to provide a desired resolution. However, this structure is not cost-effective because it does not match the resolution of human vision. Humans are more perceptive to luminance differences than to chromatic differences. Therefore, some display units map an input pixel 106 onto a subpixel repeating set that does not include the subpixels of each primary color. In such a display unit, while the chromatic resolution is reduced, the luminance resolution remains high and the reproduced image is quite acceptable to the viewer. The display unit 110 is an example of such an implementation.
The display unit 110 is a Red, Green, Blue, and White (RGBW) type, with red subpixels 120R, blue subpixels 120B, green subpixels 120G, and white subpixels 120W, wherein each subpixel is equal in area. Each set of RGBW subpixels is divided into two sets 124 of adjacent subpixels in the same row. These sets 124 are called “pairs”. Each pair 124 consists of either a red subpixel 120R or a green subpixel 120G (i.e., an RG pair) or a blue subpixel 120B and a white subpixel 120W (i.e., a BW pair). In each RG pair, the red subpixel is to the left of the green one, and in each BW pair, the blue subpixel is on the left. The RG and BW pairs alternate in each row and each column.
The pixel 106 in column x and row y of the image (i.e., pixel 106x,y) is mapped onto the subpixel pair 124 in column x and row y (i.e., subpixel pair 124x,y). Notably, in the display unit 110, the consecutive indices x and y denote consecutive pairs, not consecutive subpixels. Each subpixel pair 124 has only two subpixels, and provides a high range and resolution in luminance but not in chrominance. Therefore, part of the input pixel's luminance may have to be shifted to adjacent pairs 124 in a SubPixel Rendering (SPR) operation.
FIG. 2 illustrates a method of SPR according to the prior art.
Referring to FIG. 2, an SPR operation is illustrated for red subpixels 120R and green subpixels 120G only. The SPR operation for blue subpixels 120B and white subpixels 120W is performed in a similar manner but is not described here for sake of brevity. The SPR operation calculates the values Rw, Gw, Bw, Ww defining the luminances for the respective red subpixels 120R, green subpixels 120G, blue subpixels 120G and white subpixels 120W in a linear manner (i.e. the luminances are linear functions of the subpixel values). However, different functions may be used for different primary colors. The Rw, Gw, Bw, Ww values are then used to determine electrical signals provided to the subpixels to obtain the desired luminances.
In FIG. 2, the pixels 106 of image 104 are shown superimposed on the respective subpixel pairs 124. Again, for sake of brevity and ease of explanation, only the RG pairs 124 of the display unit 110 are illustrated. The display area is subdivided into sampling areas 250 centered at the respective RG pairs 124. The sampling areas 250 can be defined in different ways, and in FIG. 2 diamond-shaped areas 250 are chosen. The areas 250 are congruent to each other except at the edges of the display
The color of each pixel 106 is expressed in a linear RGBW color coordinate system. For each RG pair 124x,y, the Rw value of the red subpixel is determined as a weighted sum of the R coordinates of all the pixels 106 which overlap with the sampling area 250 centered at the RG pair 124x,y. The weights are chosen to add up to 1, and are proportional to the areas of overlap of the respective pixels 106 with the sampling area 250. In particular, if the subpixel pair 124x,y is not at the edge of the display, then the red value Rw is expressed by Equation (1).Rw=1/2*Rx,y+1/8*Rx−1,y+1/8*Rx+1,y+1/8*Rx,y−1+1/8*Rx,y+1  Equation (1)
In other words, the red subpixels 120R can be rendered by applying a 3×3 diamond filter to the R coordinates of the respective pixels 106 with the filter kernel of Equation (2).
                                                                0                                                      1                /                8                                                    0                                                                          1                /                8                                                                    1                /                2                                                                    1                /                8                                                                        0                                                      1                /                8                                                    0                                                                  Equation        ⁢                                  ⁢                  (          2          )                    
The same filter kernel can be used for the green, blue and white subpixels (except at the edges). Further processing may be employed, such as the use of sharpening filters to address any luminance shifts as well as gamut mapping.
While the above described SPR method provides an excellent resultant image for display, the use of the 3×3 diamond filter of Equation (2) requires the use of a plurality of row buffers in order to store information necessary for rendering each subpixel. Because each row buffer requires power for its operation, it is desirable to perform SPR using a method that reduces or eliminates the need for row buffers.
Accordingly, there is a need for an improved apparatus and method for performing SPR that provides sufficient image quality while reducing the required number of row buffers and producing a desired image.